Release 6.0.0

This release has a couple of backwards-incompatible changes:

 - the old `len()` is now `distinct_values()`
 - the new `len()` is the old `count()` (which is deprecated)
 - `IterationValue::value` became `value_iterated_to`

Some other API changes:

 - iterator values gained `quantile_iterated_to()`
 - `Histogram` gained `is_empty()`

Behind the scenes:

 - #67 and #68 landed a number of fixes to iterators such that the
   produced values are more correct and sensible.
 - errors were moved into their own module.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "hdrsample"
-version = "5.1.0"
+version = "6.0.0"
 
 description = "A port of HdrHistogram to Rust"
 readme = "README.md"